1. Technical Field
The present invention relates to an apparatus for forming an image and, more particularly, to an image forming apparatus of a type which is mounted on an electrophotographic printer and has a predetermined bias voltage-applied developing device.
2. Background Art
FIG. 3 of the accompanying drawings shows a schematic view of a typical image forming apparatus of the above mentioned type. An image formation process of this apparatus will be described below with reference to FIG. 3. As a photosensitive drum 102 rotates in a direction indicated by the arrow K, a main charger 101 charges a surface of the photosensitive drum 102 uniformly. Then, a light is irradiated onto the surface of the charged photosensitive drum 102, in correspondence with an image part, from a light exposure unit 103 which may have an LED array. At this time, the charged electric charge is removed from an exposed part corresponding to the image part (by a photoconduction phenomena) and parts other than the exposed part hold the charged electric charge whereby an electrostatic latent image is formed on the photosensitive drum 102. After that, a toner (a colored fine particle) charged to have the same polarity as the electrostatic latent image is supplied onto the surface of the photosensitive drum 102 to allow the exposed part to electrostatically absorb the toner. Accordingly, the toner image (visible image) is obtained.
Further, a paper P is fed to contact the toner image formed on the photosensitive drum 102 while the electric charge having a polarity opposite the charged electric charge of the toner is being applied onto the back face of the paper by a transferring charger 105. Consequently, an electrostatic force is produced and the toner image on the photosensitive drum 102 is transferred onto the front face of the paper P. Then, the paper P is given heat and pressure by a fixing unit (not shown). As a result, the toner image is fused on the paper P to be a permanent image. On the other hand, after the transferring, the electric charge is removed from the photosensitive drum 102 by a charge removing lamp 106. The toner still remaining on the photosensitive drum 102--some electric charge is not transferred and remains--is removed by a cleaner 107.
A high voltage is applied to the main charger 101, the developing device 104 and the transferring charger 105, respectively. A high voltage source device A applies a high voltage (about -5 KV) to a wire portion 110 of the main charger 101. In the illustration, a scorotron charger having a grid electrode 111 is used as the main charger 101. As compared with a case employing an ordinary charger, the illustrated case can reduce the unevenness of the charge potential (or charging voltage applied) on the surface of the photosensitive drum 102. Generally, about -700 V (bias voltage) is given to the grid electrode Ill to control the charge voltage at the surface of the photosensitive drum 102.
About -300 to -600 V (bias voltage) is given to the developing roller 140 of the developing unit 104 by a second high voltage source device B. The developing roller 140 is a magnet roller having a development sleeve on its surface.
A high voltage (about +5 KV) is given to a wire portion 150 of the transferring charger 105 by a third high voltage source device C.
Incidentally, the high voltage source devices A, B and C are comprised of a number of components, respectively: a DC/DC converter to accept an input from an external DC power source, a voltage raising transducer to raise a voltage of a DC pulse output from the DC/DC converter, a rectifier to rectify the voltage-raised pulse, a controller to control the DC/DC converter, the transducer and the rectifier, and other equipments. Therefore, each of these high voltage devices has a large dimension. Accordingly, the space occupied by these high voltage devices is considerably large. Further, since the high voltage devices have a large number of equipments, the cost ratio of the high voltage devices to the entire image forming apparatus is high.